02. API Publication 534 - Heat Recovery Steam Generator.PDF

A P I PUBL+534 95 m 0732290 0542770 730 m

Heat Recovery Steam Generators

API PUBLICATION 534 FIRST EDITION, JANUARY 1995

American Petroleum Institute 1220 L Street, Northwest Washington, D.C. 20005 4

COPYRIGHT American Petroleum InstituteLicensed by Information Handling ServicesCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services

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Manufacturing, Distribution and Marketing Department

API PUBLICATION 534 FIRST EDITION, JANUARY 1995

American Petroleum Institute


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I . API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPECT TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE, AND FEDERAL LAWS AND REGULATIONS SHOULD BE REVIEWED.

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Copyright O 1995 American Petroleum Institute


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FOREWORD

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict.

Suggested revisions are invited and should be submitted to the director of the Manufac- turing, Distribution and Marketing Department, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005.

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CONTENTS Page

SECTION 1 4 E N E R A L 1.1 Scope ................................................................ 1.2 Referenced Publications ................................................ 1.3 Definition of Terms ....................................................

SECTION 2-HRETUBE HEAT RECOVERY STEAM GENERATORS

2.1 General .............................................................. 2.2 Application ........................................................... 2.3 System Consideration .................................................. 2.4 Advantages of Firetube Over Watertube HRSGs .......................... 2.5 Disadvantages of Firetube Relative to Watertube HRSGs . . . . . . . . . . . . . . . . . . . 2.6 Mechanical Description ................................................ 2.7 Operations Description .................................................

SECTION 3"VERTICAL SHELL/TUBE WATERTUBE HRSGS 3.1 General .............................................................. 3.2 Application ........................................................... 3.3 System Consideration .................................................. 3.4 Advantages of Vertical Shellnube Watertube Over Firetube HRSGs ......... 3.5 Mechanical Description ................................................ 3.6 Operations Description .................................................

SECTION &WATERTUBE COILS INSIDE PRESSURE

4.1 General .............................................................. 4.2 Application ........................................................... 4.3 System Consideration .................................................. 4.4 Mechanical Description ................................................ 4.5 Operations Description .................................................

VESSELS

SECTION 5-WATERTUBE LOW PRESSURE CASING HRSG 5.1 General .............................................................. 5.2 Application ........................................................... 5.3 System Consideration .................................................. 5.4 Advantages ........................................................... 5.5 Disadvantages ........................................................ 5.6 Mechanical Description ................................................ 5.7 Operations Description .................................................

SECTION &HEAT PIPE HRSGs 6.1 General .............................................................. 6.2 Application ........................................................... 6.3 System Consideration .................................................. 6.4 Advantages ........................................................... 6.5 Disadvantages ........................................................ 6.6 Mechanical Description ................................................

APPENDIX A-STEAM DRUMS ........................................... APPENDIX B-HEAT FLUX AND CIRCULATION RATIO ................... APPENDIX C-SOOTBLOWERS ...........................................

V

~

1 1 1

2 2 2 4 5 5

12

13 13 14 15 16 16

16 16 16 17 17

18 18 18 19 19 19 25

25 26 26 28 28 28

31 37 41


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Tables 1-Extended Surface Metallurgy .......................................... 23 2-Fin Design ........................................................... 3-Maximum Gas Velocities .............................................. 25

Associated Steam Purity at Steady State Full Load Operation ............ 32 A-2-Suggested Water Quality Limits ...................................... 33 B-1-HRSG Firetube and Watertube Local Heat Flux ........................ 37

23

A-1-Watertube Boilers Recommended Boiler Water Limits and

Figures 1-Horizontal Firetube with External Drum HRSG .......................... 2 2-Vertical Firetube with External Drum HRSG ............................. 3 3-Kettle HRSG ......................................................... 4 &Insulated Metal Ferrule ................................................ 6 5-Insulated Ceramic Ferrule ............................................. 6 6"Conventional Strength Weld ........................................... 7 7-Full Depth Strength Weld .............................................. 8 &Channel-Tubesheet-Shell Interconnection ................................ 9 9-Dual Compartment Firetube HRSG ..................................... 10 10-Two Tube Pass Firetube HRSG ........................................ 10 1 I-Internal Bypass System with Valve and Damper ......................... 11 12-Partially Tubed Firetube HRSG ....................................... 12 13-Vertical Watertube Floating Head HRSG ............................... 14 1 &Bayonet Exchanger HRSG ............................................ 15 15-Pipe Coil HRSG Inside a Pressure Vessel ............................... 17 16-Basic Tubular Arrangement ........................................... 19 17-Interlaced Tubular Arrangement ....................................... 20 18-Natural Circulation HRSG ............................................ 21 19-Typical Gas Turbine Exhaust Gas HRSG ............................... 21 20-Typical Convection Section HRSG .................................... 22 21-Recommended Minimum Metal Temperature ........................... 23 22-Heat Pipe HRSG .................................................... 26 23"Operating Temperature Range for Typical Heat Pipe Working Fluids ....... 27 24-Heat Pipe Installation ................................................ 28 A-1-Typical Steam Drum ................................................ 31 B-1-Typical Watertube HRSG ............................................ 38 B-2-Typical Circulation Rate ............................................. 39 B-3-Typical Forced Circulation System ................................... 39

(Bare or Finned) .................................................... 41 C-2-Qpical Fixed-Position Rotary Mounting Arrangement .................. 42 C-3-Steam Flow Rate for Rotary Sootblowers .............................. 43 C-&Air Flow Rate for Rotary Sootblowers ................................ 44 C-5-Typical Retractable Mounting Arrangement ........................... 44

C- 1 Sootblower Cleaning Lanes for Square and Triangular Pitch Tubes

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SECTION 1-GENERAL

1.1 Scope This publication provides guidelines for the selection or

evaluation of heat recovery steam generator (HRSG) systems. Details of related equipment designs are considered only where they interact with the HRSG system design. This publication does not provide rules for design, but indicates areas that need attention and offers information and description of HRSG types available to the designer or user to aid in the selection of the appropriate HRSG system.

The HRSG systems discussed are those currently in industry use. A general description of each HRSG system begins Sections 2 through 5. Selection of an HRSG system for description does not imply that other systems are not available nor recommended. Many individual features described in these guidelines will be applicable to any type of HRSG system.

Appendices A , B, and C refer to Sections 1 through 6.

1.2 Referenced Publications 1.2.1 The editions of the following standards, codes, and specifications that are in effect at the time of publication of this publication shall, to the extent specified herein, form a part of this publication.

ABMA

1.2.2 In addition, this publication draws upon the work presented in the following publications:

Steam: Its Generation and Use, Babcock & Wilcox Company, New Orleans, Louisiana.

Combustion Engineerin- Reference Book on Fuel Burning and Steam Generation, Combustion Engineering Co., Inc., Stamford, Connecticut.

A. Bar-Cohen, Z. Ruder, and P. Griffith, Circumferential Wall Temperature Variations in Horizontal Boiler Tubes, International Journal of Multiphase Flow, Vol. 9, No. 1, Massachusetts Institute of Technology, 1983.

B.Y. Taitel and A.E. Dukler, A Model for Predicting Flow Regimen Transitions in Horizontal and Near Horizontal Gas-Liquid Flow, AICHE Journal, Vol. 22, No. 1, University of Houston, January 1976.

1.3 Definition of Terms 1.3.1 Heat recovery steam generator (HRSG)-A system in which steam is generated and may be superheated or water heated by the transfer of heat from gaseous products of combustion or other hot process fluids.

Recommended Boiler Water Limits and Associated Steam .3.2 Firetube HRSG-A shell and tube heat exchanger in

which steam is generated on the shell side by heat transferred Purity

ANSIZ/ASME from hot fluid flowing through the tubes. PTC 4.4 Gas Turbine Heat Recovery Steam Generators

Pe$ormance Test Code 1.3.3 Heat pipe HRSG-A compact heat exchanger con- sisting of a pressure vessel and a bundle of heat pipes. The heat pipes extract heat from a hot fluid and transport it into ASME3

Boiler andPressure Vessel Code, Section I , Power Boilers a where is generated. and Section W, Division 1, Pressure Vessels.

ASTM4 Standards for Tubes, Sampling, and Testing

TEMA5 Standards of the Tubular Exchanger Manufacturers

Association (seventh edition)

American Boiler Manufacturers Association, 950 North Glebe Road, Arlington, Virginia 22203. *American National Standards Institute, 11 West 42nd St., 13th Floor, New York, New York 10036-8002. 3American Society of Mechanical Engineers, 345 East 47th Street, New York, New York 10017-2392. 4American Society for Testing and Materials, 1916 Race Street, Phila- delphia, Pennsylvania 19103-1187. 5Tubular Exchanger Manufacturers Association, 25 North Broadway, Tarry- town, New York 10591-3201.

1.3.4 Vertical shell and tube watertube HRSG-A shell and tube heat exchanger in which steam is generated in the tubes by heat transferred from a hot fluid on the shell side.

1.3.5 Watertube low pressure casing HRSG-A multiple tube circuit heat exchanger within a gas-containing casing in which steam is generated inside the tubes by heat transferred from a hot gas flowing over the tubes.

1.3.6 Watertube pipe coil HRSG in a pressure vessel-A tube or pipe coil circuit within a pressure vessel in which steam is generated inside the tubes by heat transferred from a high temperature fluid or fluidized solids surrounding the tube circuits.

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2 API PUBLICATION 534

SECTION 2-FIRETUBE HEAT RECOVERY STEAM GENERATORS

2.1 General A firetube HRSG produces steam from boiler feedwater in

contact with the outside tube surface, while cooling a hot fluid which passes through the tubes. The hot fluid is often a high temperature gas resulting from combustion or other chemical reaction. Moderate temperature gases, liquids, and slumes are also used.

High temperature severe service firetube HRSGs are supplied with boiler water in substantial excess of that vapor- ized. Natural (thermosiphon) or forced (pumped) circulation systems are employed. Boiler feedwater is introduced to an overhead steam drum, which provides for water storage and steam-water separation in addition to the static head driving force for natural circulation systems.

Less severe service lower temperature firetube HRSGs are often once-through (nonrecirculating) kettle boilers. Figures 1 and 2 illustrate horizontal and vertical units involving natural circulation from an overhead drum. Figure 3 is a kettle steam generator.

2.2 Application 2.2.1 HIGH TEMPERATURUHIGH FLUX UNITS

Firetube HRSGs with high temperature process fluids (exceeding 900F) resulting in high boiling flux rates (in excess of 30,000 Btu per hour per square foot) are considered severe service applications. Gas temperatures exceeding 2000F and flux rates to 100,000 Btu per hour per square foot can be ac- commodated in firetube HRSGs. Mechanical features as described in 2.6. l are required for these severe services.

The following process applications are typical of those which often make use of severe service firetube HRSGs:

a. Steam reformer effluent (hydrogen, methanol, ammonia plants).

b. Ethylene plant furnace effluent. c. Fluid catalytic cracker flue gas. d. Sulfur plant reaction furnace effluent. e. Coal gasifier effluent. f. Sulfuric and nitric acid reaction gases.

Typical steam-side operating pressures range from as low as 150 pounds per square inch for fluid catalytic cracker and sulfur plant applications to as high as 1800 pounds per square inch for ammonia and ethylene facilities.

2.2.2 MODERATE TEMPERATURULOW FLUX UNITS

Firetube HRSGs, which handle hot fluid temperatures not exceeding 900F with flux rates of 30,000 Btu per hour per square foot and below, have a wide range of process applications. Any hot fluid stream with a temperature sufficiently above the steam saturation temperature can be utilized. Qpical process applications include:

a. Fluid catalytic cracking unit slurries. b. Miscellaneous refinery hot oil and vapor streams. c. Sulfur recovery condensers.

Steam-side operating pressures range from 50 pounds per square inch to 600 pounds per square inch.

2.3 System Consideration 2.3.1 PROCESS FLUID

The thermal-hydraulic performance and mechanical construction of the equipment to a large degree are dependent on specific characteristics of the hot process fluid. Each process fluid has unique aspects which must be accounted for in the firetube boiler design to ensure reliable operation. For example, process fluid hydrogen content may significantly increase flux.

Riser

Downcomer

Hot fluid out

Hot fluid in -W I I ( I I I I , I I

Exchanger

Figure 1-Horizontal Firetube with External Drum HRSG


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HEAT RECOVERY STEAM GENERATORS 3

2.3.1.1 Fouling

Fouling of the tube inside surface in firetube HRSGs is largely a function of the specific process fluid. It is also dependent on velocity, residence time, tube size and orientation, and wall temperature.

Examples of specific concerns include:

a. Ethylene furnace effluent quench coolers are subject to coke deposition due to continuation of the cracking process at elevated temperature. Therefore, high gas velocities resulting in minimum residence time at temperature are used. b. Hydrogen plant steam/hydrocarbon reformer effluent heat recovery boilers are subject to silica fouling when improper refractories are used in the upstream secondary reformer (for ammonia facilities), transfer lines, or boiler inlet channels. c. Fluid catalytic cracking slurry steam generators are generally designed for a velocity of 5 to 7 feet per second to avoid settling out the solid constituents. d. Fluid catalytic cracking flue gas HRSGs tend to foul with catalyst deposits.

2.3.1.2 Velocity

The fluid velocity inside the tubes must meet certain minimum criteria for the specific processes as noted under 2.3.1.1. There are also maximum velocity limitations with respect to the erosive nature of particulate bearing streams. In most cases, however, the velocity is set by maximum pressure drop or by maximum allowable heat flux limits which must be considered in design.

2.3.1.3 Pressure Drop

Pressure losses across the tube side of a firetube HRSG are limited by overall system considerations. For instance,

/ Riser , Hot fluid in

Downcomer

Figure 2-Vertical Firetube with External Drum HRSG

the performance of an olefins plant cracking furnace is pe- nalized by excessive backpressure imposed by downstream firetube quench coolers. Sulfur recovery condensers are normally designed for pressure losses of 1 pound per square inch or less, due to the low operating pressure level.

2.3.1.4 Temperature Approach

The degree to which the hot process fluid is required to approach the steam saturation temperature strongly affects the HRSG size. The approach is defined as the difference between the gas outlet temperature and the saturated steam temperature. As the design approach is reduced, the surface area requirement increases. HRSGs with large approaches tend to use larger diameter or shorter tubes than those with close approaches.

2.3.1.5 Outlet Temperature Control

Certain process applications require close control of the process fluid outlet temperature. For instance, secondary reformer effluent in an ammonia plant enters a CO to CO, shift reactor after being cooled by the firetube HRSG. Overcool- ing by the HRSG adversely affects the shift reaction catalyst. For this reason such firetube HRSGs incorporate a hot gas bypass system, which may be either internal or external. Re- fer to 2.6. l. 1 1 for further construction details.

The amount of gas bypassed is a function of turndown, extent of fouling, and the design temperature approach. The equipment tends to overcool the process fluid when run at reduced throughput and when clean. HRSGs with large design approaches tend to overcool due to the large thermal driving force at the outlet end. Such units require large bypass systems for temperature control to handle significant bypass fractions.

2.3.1.6 Gas Dew Point

Hot gas streams which may reach the dew point of one of the gas constituents require special attention. Condensation can occur on cold surfaces, such as the tubes and refractory lined walls, even though the bulk gas temperature may be above the dew point. If bulk gas cooling below the dew point occurs, as in sulfur recovery boilers, provision must be made to ensure condensate removal.

2.3.2 BOILER FEEDWATEWSTEAM

Appendices A and B provide general information with regard to the boiler feedwater/steam system. Additional considerations unique to firetube equipment are covered in 2.3.2.1 and 2.3.2.2.

2.3.2.1 Heat Flux

Maximum allowable heat flux rates for firetube HRSGs are a function of equipment construction details, steam


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pressure, recirculation rates, and water quality. Specific con- by deflector plates or dry pipes. Refer to 2.6.2.4 for addi- struction features which affect flux limits include: tional shell details.

a. Tube quantity, diameter and pitch; in general, flux limits are lower for increasing tube quantity or decreasing pitch to 2.4 Advantages of Firetube Over diameter ratio. Watertube HRSGs b. Quantity, size, and location of risers and downcomers. 2.4.1 EASE OF CLEANING c. Clearance between bundle and shell.

Tubes containing fouling prone hot process streams such Actual flux rates for comparison with design limits are as olefins plant cracking furnace effluent, coal gasifier over-

based on clean tube surface at the tube inlet where the head, and fluid catalytic cracking flue gas are easier to clean process fluid is the hottest. Firetube HRSG design should ac- in firetube HRSG~. count for increased hot process fluid heat transfer coefficients due to tube entrance effects. 2.4.2 RESIDENCE TIME

2.3.2.2 Boiler Water Circulation

Critical service high temperature firetube HRSGs are furnished with elevated steam drums, from which boiler water is supplied with substantial excess recirculation rates. Systems may be either natural (most common) or forced circulation.

Low flux HRSGs may also be furnished with an external drum. However, such HRSG equipment more commonly makes use of an expanded shell side compartment with the tube bundle submerged in the boiler water volume. Liquid disengagement occurs above the established liquid level within the expanded shell. Such a unit is commonly referred to as a kettle boiler. Natural circulation patterns occur within the kettle shell. A water-steam mixture rises through the tube bundle; the vapor rises through the steam/water interface to the steam space above; and the boiler water recirculates back down each side of the bundle to the bottom of the shell. The kettle HRSG shell serves the purposes of a steam drum in a conventional boiler system. It differs from a conventional drum in that the HRSG heating surface is self contained, connections

Firetube HRSGs have lower process fluid volume and residence time for services where time at temperature is a factor.

2.4.3 HIGH PRESSURE OR HIGH TEMPERATURE PROCESS FLUIDS OR SPECIAL METALLURGY REQUIREMENTS

High pressure process fluids contained on the tube side may minimize HRSG weight. This is particularly beneficial when alloy materials are used. For example, ammonia con- verter effluent can reach 5000 pounds per square inch and requires alloy or clad materials. For this case a firetube HRSC may be preferred.

2.4.4 VIBRATION

Firetube HRSGs are less susceptible to damaging flow- induced tube vibration or acoustic vibration when cooling large volumetric flow rate gas streams.

2.4.5 REFRACTORY LINING

are altered, and steam/water internal flow patterns are dif- Elevated temperature gas which requires insulating refrac- ferent. Saturated steam generated in kettle HRSGs is nor- tory to avoid overheating pressure bearing components is of- mally used for process or heating purposes. For such cases ten best handled in firetube equipment. This is particularly the requirements for purity and quality (see Appendix A) true for pressurized gas streams, which cannot be handled in are not high. Therefore, separation is commonly achieved rectangular duct enclosures. Refractory lining in firetube

Liquid level , / Steam space

Hot fluid in

Hot fluid out

Figure 3"Kettle HRSG


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HRSGs is generally required only in the inlet channel compartment. In comparison, shell and tube watertube HRSGs require more extensive refractory linings, which must be en- gineered to accommodate bundle insertion and removal.

2.4.6 LOW THROUGHPUT ATMOSPHERIC PRESSURE FLUE GASES

Firetube HRSGs are better suited for incinerators and other combustion systems producing relatively low flow rates of flue gas.

2.4.7 COMPACT DESIGN

Firetube HRSGs normally require less plot space due to compact design.

Horizontal firetube HRSGs with an external steam drum can have the drum mounted on the shell. The drum is supported by the interconnecting risers and downcomers, thereby eliminating costs associated with independent support.

2.5 Disadvantages of Firetube Relative to Watertube HRSGs

2.5.1 HIGH THROUGHPUT ATMOSPHERIC PRESSURE FLUE GASES

Firetube HRSGs are not well suited for handling large flow volumes of near atmospheric pressure gases. Streams such as gas turbine exhaust require large cross-sectional flow area as provided by watertube coils installed in rectangular duct enclosures.

2.5.2 LOWER HEAT TRANSFER COEFFlClENTS

Heat transfer coefficients for flow inside tubes are generally lower than for flow across the tube banks. For this reason firetube HRSGs tend to require more bare tube surface than watertube HRSGs.

The use of extended surface (fins) against a low pressure process gas can be an effective means of reducing size. This option is often utilized in watertube HRSGs, but is generally considered impractical for firetube designs.

2.5.3 HIGH PRESSURE STEAM APPLICATIONS

For cases involving high pressure steam, typically 1500 pounds per square inch and above, firetube HRSCs require heavy wall shell cylinders and tubes. This is particularly true for high capacity systems. For this reason firetube HRSGs in high pressure steam systems weigh more than their watertube counterparts.

2.5.4 HOT TUBESHEET CONSTRUCTION

The hot tubesheet design of firetube HRSGs, particularly its attachment to the shell and the tubes, may be complex.

The severity of service relates to the coexistence of multiple conditions, such as:

a. High inlet gas temperature. b. High pressure on the steam side. c. Loading imposed by the tubes due to axial differential thermal growth relative to the shell. d. Potential erosive effects of particulate bearing gases. e. Potential for corrosive attack from the process and steam sides.

The tubesheet is commonly made of Cr-Mo femtic steels which require special attention during fabrication and testing. Many firetube HRSGs require a thermal and stress analysis to prove the construction acceptable for all anticipated operating conditions.

2.6 Mechanical Description 2.6.1 HIGH TEMPERATURWHIGH FLUX UNITS

2.6.1.1 Refractory Lined Inlet Channel

Inlet channels of high temperature units are internally refractory lined to insulate the pressure components. A number of refractory systems are available, including dual and monolithic layers, cast and gunned, or with and without internal liners. Various types of refractory anchoring systems are also used. Metallic needles may be considered to further reinforce the castable.

The selection of refractory materials and their application method must be compatible with the process service conditions. The design must account for concerns such as:

a. Insulating capability, including effect of hydrogen content on the refractory thermal conductivity. b. Chemical compatibility with the process fluid. c. Gas dew point relative to cold face temperature. d. Erosion resistance against particulate bearing streams. e. Potential for coking under metallic liners.

2.6.1.2 Channels

Several channel construction options exist. The gas connections may be in-line axial or installed radially on a straight channel section. Access into the channel compartment is generally through a manway in large diameter units, or through a full access cover in small units.

2.6.1.3 Tubesheets

The single most distinguishing feature of high temperature firetube HRSGs is the thin tubesheet construction. Conven- tional shell and tube exchangers operating at moderate temperatures incorporate tubesheets designed according to the requirements of TEMA. Typical tubesheet thicknesses in such units range from 2 inches to 6 inches or more. Use of



TEMA tubesheets in high temperature high flux (severe service) firetube HRSGs is not recommended because the tubesheet metal temperature gradient would be excessive and high stresses would result.

The thin tubesheet design is based on the use of the tubes as stays to provide the necessary support for the tubesheets. Tubesheet thicknesses typically range from 5 / 8 inch to 1 /* inches. Flat portions of the tubesheets without tubes must be supported by supplementary stays.

Sufficient cooling of the tubesheet depends on efficient heat transfer at the tubesheet backface by shell side vaporiza- tion of water and high local circulation rate. This offsets the heat input from the gas through the front face and, more im- portantly, the area created by all the tube hole perforations.

The steady state tubesheet temperature is dependent on the tube pitch to diameter ratio and the tubesheet thickness.

Tubesheet temperature can be further minimized by limit- ing heat flow to the tubesheet with the use of insulated ferrules inserted in each tube inlet. The ferrules project 3 inches to 4 inches from the tubesheet face. The space between the ferrules is packed with refractory, which secures the ferrules and insulates the tubesheet face. Ferrules are either a high temperature resistant metallic or ceramic material, wrapped with an insulating paper for a lightly snug fit in the tube bore. Overcompression of the insulation will reduce its effectiveness. Figures 4 and 5 show details of one style each of a metallic and ceramic ferrule. Other configurations have been used.

Insulating material

Metal ferrule Tube

Locating pin

Refractory Tubesheet

Figure 4-Insulated Metal Ferrule

Ceramic ferrule

Refractory /

d

v Tubesheet

Figure 5-Insulated Ceramic Ferrule


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2.6.1.4 Tube-to-Tubesheet Joints

The tube-to-tubesheet joints must provide a positive seal between the process fluid and the water-steam mixture under all operating conditions and the resulting pressure and thermal loads. The joints must also withstand transient and cyclic conditions.

Tube-to-tubesheet joints in severe service applications are typically strength welded using one of the following configurations:

a. Front (tubeside) face weld The tubesheet may be J-groove beveled (see Figure 6A) or the tube may be projected from the flat face, then welded with a multiple pass fillet (see Fig- ure 6B). Additionally, each tube is pressure expanded through the thickness of the tubesheet except near the weld. Such joints may be used in elevated gas temperature applications generating steam at pressures to approximately 1000 pounds per square inch. b. Full depth weld: A deep J-groove with minimum thickness backside land is welded out with multiple passes as shown in Figure 7. If the land is consumed and fused, the tube and tubesheet become integral through the full tubesheet thickness. Full depth welded joints are often specified for high temperature gases generating steam at pressures above 1000 pounds per square inch. c. Back (shell side) face weld: This type of joint is often called an internal bore weld. The welding is performed by reaching through the tubesheet tube hole. It has been applied to a wide range of firetube HRSG operating conditions, including high pressure steam systems. A particular character- istic of this joint is that its integrity is clearly verifiable by radiographic examination.

A distinct advantage of the full depth and internal bore joints is their lack of a crevice between the tubesheet and tube outer surface. A crevice, if present, is subject to accumulation of boiler water impurities. In high temperature service the

Weld Tubesheet

J-GROOVE BEVELED TUBESHEET

Figure 6"Conventior

insulating effect of a buildup of such material can result in crevice corrosion and mechanical failure of the joint.

2.6.1.5 Tubesheet Peripheral Knuckle

A thin tubesheet is generally attached to the shell with a peripheral knuckle between the flat (tubed) portion and the point of attachment to the outer shell (see Figure 8). The knuckle provides this critical joint with necessary flexibility to absorb the axial differential movement between tubes and shell caused by operating temperatures and pressures. Proper design of the knuckle is essential for reliable operation of a firetube boiler.

The most severe cases are those involving elevated temperature gases with high heat transfer rates and with high steam side pressure. Such conditions impose considerable loads on the knuckles. An example of a severe service application would be reformer effluent in a hydrogen plant used to produce 1500 pounds per square inch steam. Exam- ples of less severe services include fluid catalytic cracking flue gas and sulfur recovery plant tail gas where condensers generate steam at 600 pounds per square inch and below.

2.6.1 -6 Channel-Tubesheet-Shell Interconnection

Numerous configurations are available for the interconnection of the thin tubesheet with the HRSG shell and the gas inlet channel. Figures 8A through 8H illustrate a number of these. Selection depends on factors such as:

a. Extent of tube versus shell differential thermal growth. b. Steam pressure. c. Process gas pressure. d. Materials of construction. e. Vertical versus horizontal HRSG orientation.

Joints shown in Figures 8A and 8B are used for mild services only, due to the fillet weld attachment and accom- panying crevice. Figures 8C through 8F all have butt

Tube Tube

Weld Tubesheet

MULTIPLE PASS FILLET

7al Strength Weld


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a API PUBLICATION 534

welded attachment to the shell. The flanged construction of Figure 8F permits channel removal. Figure 8G is used for high pressure steam service and Figure 8H is well-suited for vertically installed units.

2.6.1.7 Tubesheet Without Peripheral Knuckle Configuration

A proprietary firetube HRSG design utilizes a stiff- ened thin tubesheet which eliminates the peripheral knuckle. Rather than relieving the tube axial loads with flexible knuckles, the loads are transmitted directly to the HRSG shell through a stiffening system which backs up the thin tubesheet. This design may permit the use of longer tubes. The differential movement absorbed by the knuckles of a conventional firetube HRSG tubesheet is proportional to the tube length. For such HRSGs a length limit exists, beyond which the knuckles would be inca- pable of accepting the imposed loads within stress limits of the material.

2.6.1.8 Dual Compartment Firetube HRSGs

The length limitation described in 2.6.1.7 is of significant concern primarily with high temperature, high flux, high steam pressure equipment. For such Lases the option exists to use dual compartment construction. Two firetube HRSGs, each with conventional knuckled tubesheets, are installed in series, as shown by Figure 9.

The two compartments may be served by a common steam drum. Advantages of this configuration include:

a. Reduces differential growth between shell and tubes within each compartment. b. Permits optimization of heat transfer surface through uti- lization of different tube diameters and lengths in each compartment, thereby reducing the total surface required.

c. Permits locating the internal bypass system in the second compartment, thereby subjecting the control components to less severe temperature conditions.

2.6.1.9 Tubes

Typical tube diameters in high temperature firetube HRSGs range from 1.25 inches to 4 inches. Use of relatively large tubes permits the following:

a. Low pressure drop application typical of low pressure process gas streams such as tail gas of sulfur recovery plants. b. Thermal design at lower heat fluxes. c. Installation of tube inlet ferrules without over-restricting the flow area available at each tube entrance. d. Limits the potential for plugging tubes in services prone to fouling.

The minimum tube wall thickness is governed by applicable code rules. Except for cases involving very high process gas pressures, the steam pressure which acts externally generally controls the minimum tube thickness.

2.6.1.1 O Tube Arrangement and Spacing

Tubes are normally arranged on a triangular pattern, al-

The selection of tube pitch should address the following though square layouts may also be used.

concerns:

a. The maximum allowed heat flux is a function of the tube pitch to diameter ratio. Decreasing the pitch to diameter ratio reduces the allowable design flux. b. The tubesheet metal temperature is also dependent on the tube pitch. Decreasing the pitch increases the metal temperature. c. A minimum tubesheet ligament width between adjacent tubes is required for welded tube ends to physically accommodate the tubesheet J-groove weld preparations. This is particularly significant for full depth welded joints.

2.6.1 .ll Multiple Tube Passes

Most high temperature process firetube HRSGs are of single tube pass construction. However, multiple pass tubes may De considered for processes involving near atmospheric pressure gases used to generate low pressure steam. The low

Tube heat transfer coefficients characteristically associated with such gases result in tube metal temperatures which very

Weld closely approach the steam saturation temperature. There- fore, the metal temperature difference and differential thernlal growth of tubes of different passes are minimal. Hot pass tubes are typically larger diameter than subsequent passes in order to optimize heat transfer within pressure drop constraints. Figure 10 illustrates a two tube pass high tem-

Weld groove

Tubesheet

Figure 7-Full Depth Strength Weld perature firetube steam generator.


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Channel( tYP

Ring (typical)

Hot fluid + Hot fluid +

Tube sheet (typical)

+ E (G) (H)

Figure 8-Channel-Tubesheet-Shell Interconnection


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2.6.1.12 Gas Bypass Systems refractory lined or provided with internals to preclude the possibility of impingement of hot bypass gas on the channel

Gas bypass 'ystems for Outlet temperature wall. A typical internal bypass system is shown in Figure 11. may be external or internal to the boiler. Internal bypasses Other systems are available. are commonly used because they take advantage of cooling the bypass pipe with boiler water. The pipe maybe internally insulated to ensure that the metal temperature is maintained close to the water temperature. In high steam pressure applications the pipe may be attached to a transition knuckle in each tubesheet to absorb axial loads. The pipe is located in the center of the tube layout to provide for axisymmetric distribution of loads.

An automatically controlled valve is furnished at the outlet end of the gas bypass pipe. To reduce the size of the pipe and valve and to increase the flow control range, a plate with adjustable dampers may be installed in the outlet channel. By setting the dampers to a more closed position, the additional pressure drop imparted to the main gas stream encour- ages flow through the bypass. The outlet channel should be

2.6.1.13 Risers and Downcomers

Adequate quantity, size, and proper location of risers and downcomers are essential for reliable operation of high temperature, high flux firetube HRSGs. Setting the steam drum elevation, sizing the interconnecting circulation piping, and positioning the connections are an integral part of the design.

Riser and downcomer design and connection positioning depend on boiler orientation. Horizontal firetube HRSGs are usually furnished with multiple risers and downcomers. Connections are positioned to serve zones of equal steam generating capacity. For single pass boilers the connections tend to be more closely spaced at the hot end. At least one

Riser outlets (typical)

Intermediate channel

Downcomer inlets (typical)

Figure %Dual Compartment Firetube HRSG

r Risers Hot fluid out

+------ Downcorner

Hot fluid in "e -? \ Inlet channel Return channel 1

Figure 10-Two Tube Pass Firetube HRSG


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HEAT RECOVERY STEAM GENERATORS -

riser and downcomer pair should be located as close as possible to the hot tubesheet.

Vertical units have one or more downcomer connections located at the bottom of the boiler shell. Of greater significance, however, is the construction at the top which must ensure ample and continuous wetting of the entire shell side face of the tubesheet. The following construction options may be considered to help avoid vapor blanketing beneath the upper tubesheet:

a. Multiple riser connections installed around the full cir- cumference as high on the shell as possible. b. Reverse knuckle tubesheets to permit further elevation of the riser connections relative to the tubesheet. Refer to Figure 8H. c. Special baffling under the tubesheet to direct water across the back face of the tubesheet. d. Special formed or machined upper tubesheet with a slight taper from the center upward to the periphery. e. Installation of the entire boiler slightly canted from true vertical so that the tubesheet slopes slightly upward toward the risers which are located on that side.

2.6.2 KETTLE STEAM GENERATORS

Kettle steam generators are horizontally installed units with an enlarged shell side boiling compartment diameter relative to the tube bundle. The bundle penetrates through either a port opening in a conventional head, or the small end of an eccentric conical transition. The latter design is more common.

2.6.2.1 Tube Bundle Construction

Tube bundles may be removable or fixed. Removable bundles offer certain advantages. The bundle may be removed for inspection, cleaning, repair, or replacement. Also, removable bundles avoid the differential axial thermal expansion stress which occurs in fixed tubesheet designs.

Removable bundles may be of either U-tube or floating head construction. For fluids prone to fouling or erosive process fluids that may require mechanical cleaning or inspection, the floating head type is preferred.

2.6.2.2 Tube Size, Arrangement, and Number of Passes

Typical tube diameters are 3/4 inch and 1 inch, although larger sizes are considered for highly prone to fouling or viscous process fluids such as in sulfur condensers. Tubes are arranged on either a square or triangular pattern. The square arrangement is used if cleaning of the outside tube surface is anticipated, as could be the case for generating low pressure steam from poor quality boiler water. In such cases y4 inch minimum width cleaning lanes are maintained between tubes. Otherwise, a pitch to diameter ratio of 1.25 is normally used, unless heat flux considerations require a more extended spacing.

Multiple tube passes may be used for all bundle types described in 2.6.2.1, except for cases with extremely long hot fluid cooling ranges which may experience severe thermal stress. Single pass tubes are basically limited to fixed tubesheet construction.

2.6.2.3 Channel Construction

The selection depends primarily on the anticipated frequency of opening the unit for inspection or cleaning. If frequent access is required, a channel with bolted cover plate is desirable. Channels may be any of the TEMA designated types.

2.6.2.4 Shell Construction for De-Entrainment

A degree of disengagement of liquid is achieved in the steam space above the liquid level. The effectiveness of this volume is a function of the free height available. A typical minimum height is 20 inches in steam generating equipment. Units which produce very low pressure steam or operate at relatively high flux tend to need additional height. Simple dry pipe devices are sometimes used to enhance separation.

A properly sized kettle shell produces steam of adequate quality or purity for most process and heating applications. Higher punty steam may be achieved by the installation of separators in the vapor space above the liquid level, within a

4 Hot fluid out I

Hot fluid in --)I

API PUBLs534 95 0732270 0542787 T 3 4

API PUBLICATION 534

dome welded to the top of the kettle, or in the exit vapor line. Types of separators include:

a. Wire mesh pads. b. Chevrons. c. Cyclones. d. Combinations of Items a, b, and c.

Refer to Appendix A for further information.

2.6.3 OTHER TYPES OF FIRETUBE HRSGS

There are many other types of firetube HRSGs designed for a variety of services. They may be further classified as follows:

a. Proprietary designs developed for specific process applications. b. Boilers designed with TEMA tubesheets and external drums. The boilers may be installed in the horizontal or vertical position. c. Partially tubed horizontally installed boilers as shown in Figure 12. Tubes omitted from the top portion of the tubesheets provide the steam space for internal disengagement. The channel diameter is larger and the shell diameter is smaller than those of kettle HRSGs. Tubesheets may be TEMA, or stayed thin.

2.6.4 CODE CONSIDERATIONS

Firetube HRSGs are designed in accordance with either ASME Boiler and Pressure Vessel Code, Section I or Sec- tion VIII, Division I .

Heavy tubesheet firetube HRSGs are normally designed to TEMA requirements. ABMA guidelines are commonly followed for boiler feedwater treatment, allowable concentration of boiler water dissolved solids, blowdown, and steam purity.

2.6.5 CONSTRUCTION MATERIALS

Materials selected for use in firetube HRSGs must be compatible with the process fluid, the boiler water, and

steam with which they will come into contact. The materials must also exhibit mechanical properties consistent with the design requirements of the equipment.

2.6.5.1 Corrosion Resistance

Each process fluid from which heat is being recovered has its own composition and may therefore have its own unique requirements for construction materials. An important factor in materials selection is often resistance to hydrogen attack, because many high temperature process gas streams have significant hydrogen content. The specifica- tion of materials must also account for the possibility of gas cooling below its dew point, and the corrosive acids which may be formed. Cold metal surfaces can cause local condensation, even though the bulk gas may be above the dew point.

Pressure components wetted by boiler water, including tubes and tubesheets, are normally fabricated from ferritic materials. Boiler shells are generally carbon steel. Materials subject to stress corrosion cracking, such as austenitic stainless steels, are normally avoided and are prohibited in the evaporator by ASME Boiler and Pressure Vessel Code, Section I.

The relative growth of the shell and tubes due to temperature changes is of considerable significance to firetube HRSG design. Materials with similar coefficients of thermal expansion are beneficial. This is another reason for avoiding the use of austenitic tubing.

2.7 Operations Description Safe and reliable operation of firetube HRSGs depends on

the development and use of good operating procedures, specific to the process and HRSG design.

2.7.1 PROCESS SIDE OPERATION

2.7.1.1 New refractory lining may require a special heating sequence on start-up to effect proper dryout.

T Steam separator

Hot fluid in -+ * Figure 12"Partially Tubed Firetube HRSG


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2.7.1.2 Firetube HRSGs must not be subjected to hot gas flow unless the tube bundle is fully covered by boiler feedwater.

2.7.1.3 The rate of temperature change during transients should be controlled to minimize the potential for thermal shock.

2.7.1.4 All modes of operation should be evaluated during the design phase, particularly with regard to the ability of the boiler components to withstand the primary and secondary stresses during cyclic operation.

2.7.2 STEAM SIDE OPERATING CONCERNS

2.7.2.1 Reliability of Boiler Feedwater Supply

Of primary importance to the successful operation of firetube HRSGs is the reliable supply of boiler water to the heat transfer surface. In the event of boiler feed water supply failure, the control system must shut off the hot stream flow to the HRSG. Refer to Appendix A for further information.

2.7.2.2 Boiler Feedwater Treatment

Boiler feedwater chemical treatment must protect boiler components from water side corrosion. Improper treatment, or upsets, may cause premature failure. Water treatment specialists are normally consulted.

2.7.2.3 Continuous Blowdown

Blowdown of boiler water must be used in conjunction with boiler feedwater treatment to ensure that boiler water impurities are maintained at or below recommended maximum concentration. Continuous surface blowdown may be accomplished through a perforated collector pipe located just below the water-steam interface or a connection at the shell bottom. Continuous blowdown from kettle HRSGs should be extracted primarily from the end opposite the feedwater inlet where impurities would be most concentrated.

2.7.2.4 Intermittent Blowdown

Intermittent blowdown acts to remove settled accumula- tions of boiler water solids. Connections are located at low points in the shell, particularly in the most stagnant regions. Blowdown valves are operated at prescribed intervals, depending on the effectiveness of boiler water treatment.

2.7.2.5 Liquid Level in Kettles

There is no clearly defined water-steam interface inside the shell. Steam bubbles rise vigorously through the water from the heat transfer surfaces. A density difference exists between the two phase mixture in the boiler shell and the liquid in an external gage glass. To ensure submerged tubes, the water level is normally maintained at 2 inches to 4 inches above the top of the uppermost tube row.

SECTION 3-VERTICAL SHELVTUBE WATERTUBE HRSGS

3.1 General 3.1.1 A vertical shell/tube watertube HRSG is a vertical tube-bundle heat exchanger where steam is generated inside tubes by a shell-side hot fluid.

3.1.2 Floating head, U-tube, or bayonet and scabbard tube construction are types of vertical shell/tube watertube HRSGs. See 3.5. Figures 13 and 14 are typical floating head and bayonet/scabbard types of vertical shell/tube watertube HRSGs. Other configurations may be used.

3.1.3 Hot fluid can enter the shell from either the top or the bottom. The shell is usually a one pass shell TEMA type E arrangement. The steam/water side is a one pass system through the tube bundle or a two pass system through the U-tube and bayonet/scabbard designs.

3.1.4 The vertical shell/tube watertube HRSG can be either natural circulation (thermosiphon) or forced circulation (pumped). The U-tube arrangement is always forced circulation. For either flow arrangement, the two phase steam/water mixture is separated in a steam drum. The separated water is recirculated back to the tubes. Weight percent of steam

generated ranges from about 5 percent to about 20 percent of the total water circulated. Refer to Appendices A and B for steam drum and circulation considerations.

3.2 Application 3.2.1 Vertical shell/tube watertube HRSGs are typically used to generate steam from hydrocarbon gases or liquids, catalyst-laden flue gases, and viscous process fluids. Vertical shell/tube watertube HRSGs are generally suitable for either clean or fouling shell-side fluids. Highly fouling or very dirty fluids, however, may be better suited for tube-side designs since these types are more easily cleaned. To facilitate cleaning the shell-side of bundles in fouling or dirty service, use removable tube bundles with square or rotated square pitch and Y4 inch minimum cleaning lanes. Shell-side velocities above 1 foot per second are preferred to minimize fouling. To avoid laminar flow and low heat transfer coefficients, vertical shell/tube watertube HRSGs are used where the shell-side fluid viscosity exceeds 10 centipoise.

3.2.2 Any pressure steam may be generated, but these HRSGs are particularly suitable for high steam pressures.


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3.3 System Consideration steam pressure is 1500 pounds per square inch or greater and the process hot stream is 600 pounds per square inch or

3.3" For circu1ation7 the drum must be lower. API Recommended Practice 521 requires that the low

ating tubes. side design pressure to protect against tube rupture.

3.3.2 High purity water is particularly important for the As an alternative to designing the shell side for two-thirds

signs. Deposits in bayonet/scabbard designs might insulate overpressurization by a relief valve. The shell side design pres-

are also sensitive to deposits because of difficulty in clean- lower pressure results in lower weight. ing. See Table A- 1 for boiler water quality and associated 3.3.4 Special features are required in design or operation steam purity. to protect the HRSG in event of water failure.

3.3.3 Vertical shell/tube watertube HRSGs may weigh 3.3.5 The tube side of U-tube bundles are difficult to clean less than fire tube designs. Examples are designs where via rodding around the U-bends. The tubes are susceptible to

higher than the top tube Opening Of any Of the gener- pressure side design be raised to two-thirds the high pressure

long-term operation and reliability of bayonet/scabbard de- the high Pressure side fie shell can be protected against

the tips of the tubes causing overheating and failure. U-tubes sure can then be based On the process stream The

Steam and water out

Top tubesheet

Inlet belt

f- Hot fluid in Inlet tubes and erosion sleeves

Flanges for removable bundles

Longitudinal finned tube bundle Hot pressure shell

(refractory lined)

Bundle supports

Hot fluid out

I I

Floating head

t for expansion BFW in

Figure 13-Vertical Watertube Floating Head HRSG


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scale, deposits, and debris because of the small inside tube diameter and orientation of the tubes. Such debris may col- lect at the low point in the U-bend. Downcomer strainers may be installed to minimize tube plugging from large for- eign objects.

Bayonet tube designs have a restrictive annulus between the tubes. This requires extra care in keeping debris out of the system which could block flow to a tube. A strainer is generally required in the water supply piping at the steam generator entrance in these cases.

3.4 Advantages of Vertical ShelVTube Watertube Over Firetube HRSGs

3.4.1 Vertical shellhube watertube HRSGs can be designed for higher maximum heat flux than firetube HRSGs since flow distribution and steam generation are

more uniform. See Table B-1 for heat flux comparison of watertube HRSGs versus firetube HRSGs.

3.4.2 If multiple risers are required, they can each be arranged to have the same proportion of total flow. In high pressure systems, equal flow in each riser provides more uniform distribution to facilitate better performance of the steam drum.

3.4.3 The tube bundle is free to expand so stresses from differential expansion between the tubes and shell are not large. This is particularly true of the bayonet tube design where each tube can expand independently without inducing tubesheet stresses from differential expansion between adjacent tubes.

3.4.4 The vertical shell/tube watertube HRSG requires no refractory lining of the tube side channel or floating head

BFW inlet + Top tubesheet

Flanges for removable bundles

Bayonet

Scabbard

Cap

-L c

c

c

c

J

Hot fluid in I -

Steam and water out

Hot fluid out

support lugs

Cross flow baffles

Pressure shell

Figure 14Bayonet Exchanger HRSG


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tubesheet face. In the event of a single pass bare tube failure, 3.5.3 U-TUBES no refractory has to be removed to repair the failed tube.

3.4.5 If U-tube and bayonet tube designs have tubesheets at the outlet (cold end) of the shell, the tubesheets can be designed with lower alloy metallurgy than when located at the inlet or hot end of the shell.

3.4.6 In the event of tube failure for bayonet tube units, the bayonet tube bundle and tubesheet can be removed to access the scabbard tube or tubes that failed.

"

A U-tube vertical shell/tube watertube HRSG consists of a vertical shell with a channel at the top. Inside is a tradi- tional U-tube bundle. Water enters the channel, flows down the inlet leg and up the outlet leg of the U-tube. The heating fluid flows up or down through the bundle shell side. The U- tube bundle is free to move in response to thermal growth and is not affected by the shell thermal growth.

3.5.4 BAYONET TUBES

3.4.7 Spare tube bundles will enable quick turnaround The bayonet vertical shell/tube watertube HRSG employs since the damaged bundle can be quickly pulled and the a set of concentric tubes. The smaller diameter tube is called spare bundle installed to allow the unit to be put back on the bayonet, and the larger diameter tube is called the scab- stream. bard. The scabbard tube pattern may be a standard triangular

or square pitch. Each scabbard tube is inserted through two 3.5 Mechanical Description tubesheet holes and attached. The end of the scabbard tube is 3.5.1 FLOATING HEAD

The bottom tubesheet is a floating head configuration with a nozzle that passes from the floating head through the shell head to allow water to be fed from the bottom. The feed water is heated and becomes a two-phase mixture as it passes through the heated zone and enters the top channel. After one pass through the exchanger, the steam/water enters a steam drum. The hot fluid may be concurrent or countercur- rent on the shell side. An expansion joint in the floating head inlet nozzle and the floating head are provided to compen- sate for thermal expansion between shell and tube.

3.5.2 FIXED TUBESHEET

capped. The bayonet tube and tubesheet pattern are identical to the

scabbard pattern. The bayonet tubes are attached to this tubesheet and are not capped.

The two bundles are assembled so that each bayonet is inserted into its corresponding scabbard. The tubesheets are separated axially by a channel spacer piece. Refer to Figure 14.

3.6 Operations Description The boiler feed water enters the bayonet channel compart-

ment, flows down through the bayonets, up the scabbard annulus to the lower channel compartment, and then out into a steam/water riser. The double channel bundle is inserted into a vertical flanged shell. The process heating stream enters

The shell of the heat exchanger may contain an expansion the shell at the bottom below the ends of the scabbard bundle joint to allow for differential thermal growth between the and flows up through the bundle, exiting the shell at the top tubes and the shell. just beneath the scabbard bundle tubesheets.

SECTION 4-WATERTUBE COILS INSIDE PRESSURE VESSELS

4.1 General 1400F. This catalyst is a large source of heat to generate steam within the coil(s).

4.1.1 Watertube coils may be located either horizontally or vertically inside a vessel. The vessel contents are the heat 4.3 System Consideration source. Figure 15 shows a horizontal arrangement.

4.3.1 Frequently the coils are designed with multiple tube 4.1.2 The coils are supported inside the vessel at several side flow passes. In these cases, the cross-section flow area, parallel levels. the heat transfer surface area of each pass, and the water

flow rate to each pass should be identical. Design inlet flow

second.

4.3.2 The heat transfer parameters, such as transfer rate, A typical example is a cat-cracker catalyst regenerator circulation ratio, and specific flow rate, must be selected to

4-1 -3 The coils may be fabricated of either pipe Or tubing. velocity in each pass should be in the range of 3 to 7 feet per

4.2 Application

equipped with a fluidized bed of catalyst at 1300F to ensure nucleate boiling.


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4.3.3 The design of the coils is quite critical. Conservative practice dictates careful choice of coil inside diameter, length, and heat flux to arrive at a satisfactory and operable system to avoid film boiling, vapor lock, or unstable flow conditions. During process upsets, such as cessation of bed fluidization or circulation, loss of steam generation may occur in one or more of the individual parallel passes. The system design shall consider this possibility.

4.3.4 Forced circulation is recommended to ensure a fixed flow rate to each coil with a minimum circulation ratio of 5: l .

4.3.5 An annular flow regime is desired. If annular flow cannot be achieved, special inserts may be installed to ensure wetted walls.

4.3.6 The system shall be designed with the water side pressure always being greater than the shell side pressure.

4.3.7 Applications may exist where flow measurement or control may be required on individual water side passes of multipass coils. Isolation of individual passes may be considered, although the loss of cooling may damage the coil.

4.4 Mechanical Description The coil shall be designed to meet the requirements of the

applicable ASME code. Thermal stresses and mechanical loads must be considered in designing the coils.

4.5 Operations Description 4.5.1 Proper drum water treatment must be exercised to avoid having water side deposits. Such deposits would lead to high tube metal temperatures, loss of thermal efficiency, and ultimate tube failure. Water side deposits may also lead to possible pump failure. Where several parallel steam generation coils are required to make up a total steam rate, a careful choice of control valve location is necessary to ensure balanced flow. Since the heat transfer environment in which the steam generating coils are located is a severe service, orifices balance flow at the inlet of each parallel coil. Control valves must be provided there also to shut down specific coils in the event of a break or leak from the coil.

4.5.2 Pressure relieving devices may be required on the vessel to protect the vessel in the event of coil failure.

I

Vessel wall

I

LL-l

COIL SUPPORT

PLAN VIEW COllS

I ELEVATION VIEW

Figure 15-Pipe Coil HRSG Inside a Pressure Vessel


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SECTION 5-WATERTUBE LOW PRESSURE CASING HRSG

5.1 General pressure steam for injection into combustion turbines for 5.1.1 TYPICAL SYSTEM

The watertube low pressure casing HRSG generates steam inside a number of tube circuits that are heated by a hot gas stream flowing through an enclosure of insulated steel casing plate.

The gas normally flows across the tubes in a single pass from inlet to outlet. In certain cases, baffles or directional vanes may be used to direct the gas across the tubes creating additional gas passes.

Steam generating tubes are connected to drums or headers. The tubes may be arranged in one continuous circuit or may be manifolded at their inlet and outlet ends to form a number of parallel flow paths.

Steam drums may be either integral to the steam generating tube circuit or mounted remotely from the tubes.

Additional tube circuits may be used for preheating feedwater or superheating steam.

5.2 Application The watertube low pressure casing HRSG is used to re-

cover heat from low pressure exhaust or flue gases. Some common applications are:

a. Heat recovery from combustion turbine exhaust gas to produce steam for use in process(es), in enhanced oil recovery, and in cogeneration. b. Heat recovery from fired heater flue gas to produce steam. c. Heat recovery from fluid catalytic cracking regenerator flue gas to produce process steam.

The casing shall be designed with tight joints, preferably seal welded, to prevent gas leaks to the atmosphere. Some minor leakage may occur at casing penetrations where thermal growth must be accomodated. The watertube low pressure casing HRSG is not used in services where leaks of the exhaust or flue gas would not be permissible.

5.3 System Consideration The watertube low pressure casing HRSG may generate

steam at a single pressure level or multiple pressure levels. Generating at multiple pressure levels may increase the effectiveness of recovering the heat from the gas and will provide steam for various plant needs.

Combustion turbine HRSGs used in cogeneration or com- bined cycle power plants often generate steam at two or three pressure levels. Each pressure level may have its own economizer, evaporator, or superheater section.

Multiple pressure level combustion turbine HRSGs often produce high pressure steam for steam turbines, intermediate

NO, control or induction into steam turbines, and low pressure steam for deaerators.

When steam requirements are greater than can be obtained with the heat available from a combustion turbine exhaust gas, supplementary firing can be used to increase the available heat.

If steam is required when the combustion turbine gas flow is curtailed, fresh-air firing (see 5.6.11) with ambient combustion air can be used to generate steam.

A gas bypass system can be provided to isolate the gas source from the HRSG. This will allow independent operation of the gas source or HRSG. A combustion turbine can be started with the gas bypassed to the atmosphere for rapid start- up. Modulating the gas can also reduce the thermal shock of start-up to the HRSG or control the HRSG steam output.

The gas may be slightly above or below atmospheric pressure. Combustion turbine exhaust static pressures are typically in the range of 8 inches to 16 inches H20 and the HRSG casing design pressure is typically 20 inches H20. Fired heater flue gas pressures are normally slightly below atmospheric pressure. Fluid catalytic cracking regenerator flue gas pressure may be 1 pound per square inch gauge or greater. The HRSG casing is normally limited to gas pressures of 5 pounds per square inch gauge or less due to the practical strength of the gas path enclosure casing.

For combustion turbine Hk'SGs, the gas pressure drop should be optimized considering the reduction in combustion turbine output with increased back pressure and the enhanced heat recovery with increased gas pressure drop in the HRSG.

Combustion turbine exhaust gas temperature may range from 750F to 1 lOOOF. When supplementary fired, the gas temperature entering the HRSG may be as high as 1800F without water wall construction.

The gas temperature exiting the HRSG is normally in the range of 250F to 500F. The exit gas temperature depends on feedwater temperatures, steam requirements, and corrosion considerations. The exit gas temperature should be above the water and acid dew point of the gas to avoid excessive corrosion of the casing.

To prevent corrosion, the temperature of tube and fin surfaces exposed to the gas should be maintained above the water and acid dew points unless special metallurgy is used.

Feedwater preheat sections are sometimes bypassed (run- dry) when firing oil to prevent corrosion of the section.

Feedwater preheat coils may contain water that has not been deaerated. Tube metallurgy other than carbon steel may be required.

For environmental consideration, HRSGs are often required to contain emission control equipment such as selective catalytic reduction systems for NO, control.


API PUBLS534 95 m 0732290 0542794 L74 m ~~ ~-


Emission control systems are located within the HRSG such that the flue gas temperature range in the system is correct for a selective catalytic reduction system operating temperature range. The location and operating temperature range is influenced by gas turbine/supplementary firing modes of operation.

When fouling is anticipated, cleaning provisions may be required. The configuration of the heat transfer surface may have to be modified to enhance cleaning.

Protection from freezing is especially important for HRSGs used in cyclic service. Provisions for rapid start-up may also be included such as steam sparging or steam coil heating in drums to maintain the pressure components in a warm condition.

5.4 Advantages The HRSG can recover energy otherwise exhausted to the

atmosphere rather than burning fuel in a fired boiler, reducing plant fuel consumption and emissions.

The low pressure steel casing design permits very large units that can handle high volumes of hot gases and produce large quantities of steam.

The HRSG is capable of rapid start-up. The configuration may be varied to suit plant equipment

and plot requirements. Different process steam users can be supplied from one piece of equipment.

Emission control equipment can be incorporated in the low pressure casing HRSG at optimum operating temperatures. Other HRSG types may not have this capability.

5.5 Disadvantages The watertube low pressure casing HRSG is not suitable

The HRSG casing is limited to designs requiring 5 pounds

HRSGs not enclosed in buildings must be winterized

for small gas volumes.

per square inch gauge or lower.

when installed in cold climate locations.

5.6 Mechanical Description 5.6.1 HORIZONTAL TUBE EVAPORATOR

The flow within a horizontal tube evaporator normally is forced circulation as described in Appendix B, and Figures 16 and 17. The steam drum is mounted remotely from the tubes.

It is possible to establish natural circulation through horizontal tubes by elevating the water outlet from the steam drum sufficiently above the tubes. However, hydraulic resistance and vapor blanketing in the tubes are potential problems. Forced circulation flow is generally preferred for horizontal tubes.

5.6.2 VERTICAL TUBE EVAPORATOR

The flow within a vertical tube evaporator normally is natural circulation as described in Appendix B and Figures 18 and 19. Downcomers can be external to the gas stream con- necting the upper drum and lower drum. Downcomers can also be located within the gas stream. Circulation rates must consider heat input to an internal downcomer.

I I I + I 1 Feed water e Steam generator

I - 1 J - 1 L

- 1 1 I

I Steam superheater

t t t t Gas

Figure 16-Basic Tubular Arrangement


20

A P I PUBL*534 75 m 0732290 0542795 O00 m

API PUBLICATION 534

5.6.3 INCLINED TUBE EVAPORATOR

The flow within inclined tube evaporator arrangements is from a lower drum or header upward through parallel inclined tubes to a collector drum, header, or the steam drum. Natural circulation is utilized, similar to that described for vertical tubes. The slope of the tubes and the configuration of the drums, headers, tubes, and exhaust gas path is critical to proper operation of an inclined tube arrangement.

5.6.4 PREHEATERS, ECONOMIZERS, AND SUPERHEATERS

In addition to the evaporator, the HRSG may include an economizer section to heat the feedwater and/or a superheater section for superheating steam. (See Figures 16, 17, and 19). Multiple pressure level HRSGs may have economizers or superheaters for more than one pressure level.

A steam reheater, feedwater preheater, or a deaerated steam generating coil may be included at appropriate exhaust gas temperature locations.

The exhaust gas normally passes over the superheater, steam generator, and economizer (in that order) to optimize the heat transfer effectiveness of the HRSG. Alternative arrangements may be used, such as integrating economizers, evaporators, and superheaters of different pressure levels to optimize heat transfer rather than locating the sections of each pressure level together. Superheater sections may be located within steam generator sections to limit superheater

tube metal temperatures or the variation of superheated steam temperature with variations in gas flow or temperature.

5.6.5 STRUCTURE AND CASING

The gas path enclosure is normally a rectangular box enclosed on four sides with steel casing plate and open on both ends for entry and exit of the gas stream. Figures 19 and 20 are examples of gas turbine exhaust and fired heater HRSG layouts, respectively. Transition ducts are normally provided at the inlet and exit of the HRSG for connection to the gas source and to a duct or stack exhausting to the atmosphere or gas cleaning system.

An external structural framework integral to the casing plate enclosure supports the tubes, headers, and steam drums. Vertical tubes and their drums or headers may be supported either from the top or bottom of the structure. Horizontal tubes may be supported by intermediate and end tube supports.

The gas path enclosure casing plate may be an internal casing or an external casing depending on whether thermal insulation is installed on the outside or inside of the casing plate.

External pressure casing designs are suitable for gas temperatures to 1800F. The carbon steel casing is protected from hot gas temperatures by the internal insulation. External casing designs are suitable for rapid start-up, with start-up rates dependent on the type of internal insulation system used.

Less common, internal casing designs of carbon steel may be suitable for gas temperature of approximately 750F or

L

- 1 1 I

I 1 I

3 I

Feed water

I! Steam superheater LL W

u) W

c

9

m 1 I

_ I c

Steam generator I

- 1

Gas

Figure 17-Interlaced Tubular Arrangement


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Steam

Feed water-

Feed water

Figure 18-Natural Circulation HRSG

Gas out

Headers

Pressure casing

Water in

SUPERHEATER EVAPORATOR ECONOMIZER

Figure 19-Typical Gas Turbine Exhaust Gas HRSG


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lower. Thermal expansion stresses, thermal growth, and ac- celerated corrosion of the casing plate are considerations for internal casing designs. Carbon steel casing plate with increased copper content such as ASTM A 242 or A 588 may be used to improve corrosion resistance. Rapid start-up with internal casing designs must consider expansion capabilities due to the thermal stresses developed between the casing and structural supports or insulation.

5.6.6 INSULATION

Insulation is provided on the casing to minimize heat loss and provide personnel protection. If the gas temperature is above 750F, an internal insulation system is normally used to protect the steel casing plate.

External insulation is normally block or blanket insulation with metal lagging for weather protection.

Internal insulation may be castable refractory, ceramic fiber, mineral wool block, or blanket insulation. '

Internal castable refractory is normally used for fired heater or fluid catalytic cracking regenerator HRSGs. It is suitable for high gas velocities and resists erosion from tube cleaning devices. Various types of castable refractories are available for resistance to gas erosion and chemical attack. Castable refractories are suitable for HRSGs, particularly when the firing duct temperature exceeds 1600"E

Internal block or blanket insulation is normally used for combustion turbine HRSGs. The insulation is normally a layered construction with the steel casing on the outside and a system of steel liner plates on the inside to prevent insulation damage from gas flow velocities or tube

cleaning devices. The internal steel liner is constructed with panels overlapping in the direction of exhaust flow held in place by support pins and washers on the inside surface of the liner. The panels accommodate thermal expansion because the edges are free to slide over each other. Liner materials should be selected for gas temperatures and corrosion characteristics.

5.6.7 TUBES

Horizontal tube steam generators normally consist of multiple rows of tubes ranging in size from l inch to 4.5 inches outside diameter. The tubes may be headered into parallel passes. Tubes within the same pass are connected in series by return bends.

Natural circulation vertical tube steam generators normally consist of a group of parallel single tube circuits. Each tube is connected to the upper drum or header and a lower drum or header. Water enters each tube at the bottom and flows unobstructed vertically to the top drum or header. Tubes are normally 2 inches at the outside diameter.

Forced circulation vertical tube steam generators are commonly used for fluid catalytic cracking regenerator HRSGs. Multiple tube passes are used between the inlet and outlet headers. Tubes are connected with return bends.

The normal spacing between centers of adjacent tubes is two times the nominal tube diameter. The tubes in adjacent rows in the gas path may be arranged in an in-line or staggered pattern. For the same gas flow, staggered arrangements improve heat transfer over in-line arrangements, but gas pressure drop is increased.

External casing

Steam/water out

Water in

Manifold

lnsulation/refractory

C- Header box

Sootblower lane

Sootblower

Intermediate tube support

Gas

Figure 20-Typical Convection Section HRSG


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Gas side extended surface is used to increase the heat transfer to the tubes. Welded spiral wound solid or serrated fins are normally used. Studs may be used when extreme fouling is expected.

The metallurgy of extended surface is selected for resistance to high temperature oxidation. Recommended maximum temperature for various materials are shown in Table l.

A minimum temperature limit results from corrosion requirements. Keep the cold end metal temperature above the exhaust gas dewpoint to prevent acid attack of the metal. Metal temperatures required to avoid condensation and the resulting corrosion are s

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02. API Publication 534 - Heat Recovery Steam Generator.PDF

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